Memory manufacturers are currently researching and developing the next generation of memory devices. One such development includes technology designed to replace current volatile and non-volatile memory technologies. Important elements of a successor include compactness, low price, low power operation, non-volatility, high density, fast read and write cycles, and long life.
Current memory technology is predicted to survive into 65 nanometer process generations. This survival is in part based on the successful integration of, for example, exotic storage, source and drain engineering, copper and low dielectric constant materials for the interconnect levels, and high dielectric constant materials for transistor gates. However, there will thereafter exist a need for new memory materials and technology, particularly for non-volatile memory.
As is well known in the art, Flash memory utilizes a floating gate to store charge indicative of a logical “0” or logical “1” memory state. The floating gate is located between a control gate and a substrate, and relies on hot electron injection and Fowler-Nordheim electron tunneling through a thin tunneling oxide between the floating gate and the substrate for charge injection. An electrical potential, usually between 10 and 13 volts, can be applied to the control gate to excite electrons through the tunneling oxide layer into the floating gate where they are thereafter trapped. The trapped electrons provide excess potential in addition to the potential applied at the control gate. Hence the current through the transistor channel in the substrate is a function of both the control gate voltage and the presence/absence of charge in the floating gate. In other words, the compounded effect of the stored charge and the control gate voltage sets the resistance in the current channel, controlling the current flow through it. A cell sensor (external circuitry) monitors the potential drop across the current channel in the substrate which is controlled by the resistance of the channel to the current flow. If, for example, the resistance through the gate is greater than a set threshold value in Ohms, it has a logical value of “1.” If the resistance drops below the threshold, the logical value changes to “0.” The non-volatility of the memory depends on how securely the electrons are trapped in the floating gate. Among other defects, weak spots in the tunneling oxide (in particular as the tunneling oxide thickness decreases) may enable a filament current that will discharge the entire floating gate and render the device useless as a non-volatile memory element as the floating gate will be unable to store charge for any useful duration which leads to product reliability concerns.
Nanocrystals have been introduced as a paradigm to increase tunneling oxide reliability of Flash memory by dividing a monolithic floating gate into a set of discretely spaced floating gates. In the event of a weak spot or defect in the tunneling dielectric, this discreteness allows the discharge of only the floating gate directly over the defect. The rest of the floating gates are unaffected by the defect instead of the catastrophic leakage of all stored charge from a monolithic floating gate.
Another paradigm shift involves the use of carbon nanotubes in electronic applications. In particular, single-walled carbon nanotubes (SWNTs) are nanometer scale cylindrical tubes that are rolled from a single graphene sheet that can either be grown from a carbon source with the help of a catalyst. Nanotubes can have various crystal orientations and diameters which produces a variety of electronic band structures. Thus, SWNT can either metallic or semiconducting. As a semiconductor, a SWNT or multiple SWNTs can replace the semiconductor (e.g., silicon substrate) in a metal oxide semiconductor field effect transistor (MOSFET) structure. Such devices are also called carbon nanotube field effect transistors (CNFETs). However, while the promise of SWNTs in electronics applications theoretically impressive, SWNT-based electronic manufacturability offers significant hurdles to commercial practicability.